Vehicles with internal combustion engines typically utilize emission control devices, such as catalytic converters, to reduce exhaust emissions. In one example vehicle system, multiple catalytic converters are utilized, with one in an upstream location and one in a downstream location. Further, the performance of these emission control devices is monitored using exhaust gas sensors. If degradation of the emission control device is detected, then an indicator light it typically illuminated.
The inventors of the subject application have recognized an apparently irreconcilable paradox in designing an emission control system that meets the ever-increasing emission control requirements. In particular, the inventors have found that for improved air-fuel control using multiple exhaust gas sensors, it is good to have larger amounts of catalyst volume between the sensors, and a smaller amount of catalyst volume downstream of the last exhaust gas sensor. This allows for improved air-fuel control and reduced emissions, even as the catalysts and exhaust gas sensors age over time.
On the other hand, the inventors herein have recognized that for improved diagnosis, it is good to have a small catalyst volume between exhaust gas sensors. This is due in part to the strict emission regulations. Specifically, if the upstream catalyst is degraded, the emission requirements may not be met even if the downstream catalyst is functioning at peak efficiency. In other words, diagnostics that monitor the upstream catalyst performance independent of the downstream catalyst performance can result in improved diagnostics.
As such, some systems can require three exhaust gas sensors to meet these competing objectives. However, the inventors herein have recognized that this can add significant cost to the vehicle, since only two sensors were used in previous vehicles.
The disadvantages of prior art approaches are a overcome by a computer-controlled emission diagnostic system for an engine exhaust system of a vehicle, comprising a first emission control device coupled to an exhaust of the engine; a second emission control device coupled downstream of said first emission control device; a first exhaust gas sensor coupled upstream of said first emission control device; a second exhaust gas sensor coupled in or downstream of said second emission control device; a computer storage medium having a computer program encoded therein for monitoring the engine exhaust system comprising: code for reading a first signal from said first exhaust gas sensor, code for reading a second signal from said second exhaust gas sensor; code for indicating an interval in which said first emission control device is operating above a preselected performance and said second emission control device is operating below a predetermined performance based on operating conditions; and code for determining performance of said first emission control device based at least on said first and second signals during said indicated interval.
In one example, the interval occurs directly after an engine cold start, where the first emission control device warms and becomes active before the second emission control devices can warm and become active.
By determining performance during the interval when the upstream device is operating with a specified performance while the downstream device is not, it is possible to perform accurate diagnosis even with two sensors having sufficient volume between them so that accurate air-fuel control can be achieved. In other words, during the specified conditions, the upstream device should be performing substantially more emission conversion than the downstream device. As such, even though two sensors are used with both devices between them, it is possible to determine the performance of the upstream device. At the same time, the sensors are in an effective position to provide accurate air fuel ratio control. As such, it is possible to provide a system without requiring additional sensors.
Note that, if desired, additional exhaust gas oxygen sensors can be used to further supplement the system. Also, it is possible to have additional emission control devices, which can be added in an upstream and/or downstream location. Note also that there are various ways to determine the interval, such as time duration, er based on estimates of catalyst temperature or storage capacity.